1. Field of Invention
The present invention relates to, and finds utility within, wireless information communications systems. More particularly, the present invention relates to diversity reception of downlink signals at the handset without requiring dual receive chains.
2. Related Art
Wireless radio telecommunications systems enable many mobile users or subscribers to connect to land-based wire-line telephone systems and/or digital Internet service providers enabling access to the World Wide Web digital information backbone. Conventional wireless air-interfaces include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA), and improvements therein.
The CDMA air-interface calls for modulation of each carrier with a unique pseudorandom (pseudo-noise) code. As the CDMA users simultaneously occupy the same frequency band, the aggregate data signal transmitted by a fixed base station (forward link) is noise-like. A common pilot tone is transmitted to all mobiles within the effective service area of the base station. Individual signals are extracted at the mobile by correlation processing timed by the pilot tone.
The CDMA air-interface is in a state of constant improvement. A latest iteration of the CDMA standard is known as xe2x80x9cthird generationxe2x80x9d or xe2x80x9c3Gxe2x80x9d. For digital data traffic one proposed solution for CDMA 3G is known as xe2x80x9cCDMA/HDRxe2x80x9d, or simply xe2x80x9cHDRxe2x80x9d. HDR uses known techniques to measure channel data transfer rate, to carry out channel control, and to mitigate and suppress channel interference. One approach of this type is more particularly described in a paper by Paul Bender, Peter Black, Matthew Grob, Robert Padovani, Nagabhushana Sindhushayana and Andrew Viterbi, entitled: xe2x80x9cCDMA/HDR: A Bandwidth Efficient High Speed Wireless Data Service for Nomadic Usersxe2x80x9d, published on the internet at the time of filing of this application by Qualcomm Corporation at the following URL: xe2x80x9chttp://www.qua1comm.com/hdr/pdfs/CDMA_HDR_IEEE.pdfxe2x80x9d. The disclosure of this article is incorporated herein in its entirety by this reference thereto. Principles articulated in this paper are being incorporated into a draft 3G specification, known as xe2x80x9cDraft Baseline Text for the Physical Layer Portion of the 1xc3x97EV Specificationxe2x80x9d being proposed within the Third Generation Partnership Project Two (3GPP2), the disclosure thereof being incorporated herein in its entirety by this reference thereto.
In the 1xc3x97EV approach described in the above draft specification, each mobile station measures the received signal-to-interference-plus-noise ratio (SINR) based on the received common pilot sent out by the base station. The data rate which can be handled by the particular mobile is proportional to its SINR. Therefore, the mobile will repetitively determine forward link SINR and communicate a maximum supportable data rate back to the base station via the mobile""s reverse link channel.
FIG. 1 graphs the 1xc3x97EV forward link (base station to mobile unit) traffic channel, as well as reverse link data rate control channel and acknowledgement channel along a common time base. In accordance with the 1xc3x97EV specification, forward link traffic channel and reverse link control channel physical layer packets can be transmitted in 1 to 16 time slots, with each time slot being 1.66 milliseconds in duration at a data rate of 153.6 kbps. When more than one time slot is allocated to a subscriber, the forward link transmit slots use a 4-slot interlace. That is, adjacent transmit slots of a particular 4096 bit traffic packet are separated by three intervening slots, and slots of other packets are transmitted in the slots between those transmitted slots. If a positive acknowledgement is received on the reverse link ACK channel that the physical layer packet has been received on the forward link traffic channel before all of the allocated slots have been transmitted, the remaining untransmitted slots will not be transmitted and the next allocated slot is used for the first slot of the next physical layer packet transmission.
Diversity reception is a recognized technique to reduce effects of signal fading and/or co-channel interference. In this method, a resultant signal is obtained by a combination or selection, or both, of two or more sources of received-signal energy that carry the same modulation or user information content (xe2x80x9ctrafficxe2x80x9d), but may differ in signal strength, or signal to interference plus noise (SINR), at any given instant. For example, a base station may transmit the same traffic simultaneously via two separate frequencies. Two receive chains of a mobile unit then provide the required two sources of received-signal energy for combinatorial diversity reception with each chain tuned to a respective one of the frequencies.
Open loop diversity is proposed for CDMA 3G air-interfaces, for example xe2x80x9corthogonal transmission diversityxe2x80x9d or xe2x80x9cOTDxe2x80x9d. In the OTD approach, the base station includes two transmit channels for transmitting simultaneously two coded signals via two spatially separated antennas. Each channel is coded with a unique Walsh code so that its information content or xe2x80x9ctrafficxe2x80x9d is orthogonally related to the other channel""s otherwise identical traffic. The mobile station or handset simultaneously receives the two signals at its antenna, amplitude and phase matches one of the received signals to the other via a RAKE receiver, and decodes the two Walsh coded channels, thereby enabling the information content of the channels to be combined in proper amplitude and phase in order to reduce effects of fading and co-channel interference. A block diagram depicting a base station BTS 10 having two transmit channels 12 and 14 is set forth in FIG. 2. Channel 12 includes an antenna or antenna array 16, and channel 14 includes an antenna or antenna array 18. Antennas 16 and 18 are, e.g., spatially separated. Antenna 16 transmits a signal over path 20 and antenna 18 transmits a signal over path 22.
Signal paths 20 and 22 arrive at an antenna 24 of a handset 26. Signal path 20 includes the forward link traffic and first interference, for example, and signal path 22 includes the same forward link traffic (coded to be orthogonal with respect to the traffic on path 20) and second interference. A RAKE receiver within handset 26 separates the two channels by virtue of the orthogonality of the Walsh coding, amplitude matches and phase matches the two channels, and combines the two channels together in a manner providing diversity reception.
As illustrated by FIG. 2, the OTD approach requires providing more transmit channels at the base station 10. If the base station 10 includes an antenna array for beam forming in order to provide coherent spatial gain at the mobile station or handset, implementing the OTD method requires two antenna arrays, thereby doubling the number of transmit channels, or reducing potential coherent gain by about 3 dB. One potential drawback of the OTD method is that each subscriber unit, e.g. handset 26, being serviced within the particular service sector requires two Walsh codes. While the OTD method proposes using QPSK modulation in order to double the number of available Walsh codes, there are ultimately only a finite number of available Walsh codes. Therefore, a base station potentially becomes capacity-limited to one half of the number of subscribers that can be simultaneously served with an air-interface using only a single Walsh code per subscriber unit, given the same finite number of available Walsh codes. Another drawback found with the OTD method is that if two channels and antennas are used to transmit from the base station, for much of the time the two channels will remain highly correlated at the handset location, and the advantages otherwise provided by diversity reception will not be realized.
One example of a receiver architecture for concurrent diversity reception is provided by U.S. Pat. No. 6,014,570 to co-inventor Piu Bill Wong and Donald Cox, entitled: xe2x80x9cEfficient Radio Signal Diversity Combining Using a Small Set of Discrete Amplitude and Phase Weightsxe2x80x9d, the disclosure thereof being incorporated herein by reference. In the approach described in the ""570 patent, a handset includes two antennas. A first antenna receives a first signal, and a second antenna receives a second signal. The antennas are separated by spatial, polarization and/or pattern separation. Coupled to at least one of the antennas is a circuit for introducing a complex weight xe2x80x9cAxe2x80x9d having a gain component for varying the gain and a phase component for varying the phase of the signal received at the antenna to match the gain and phase of the signal received at the other antenna. The gain-matched and phase-matched signals are then combined in a combiner circuit within the receiver. In this prior approach, the gain and phase are constrained to be selected from a finite set of preselected discrete gains and a finite set of preselected discrete phases. While the approach of the ""570 patent works well in reducing co-channel interference, it is hampered by its relative complexity and by a need to include two full receive channels, as well as two antennas, at the receiving location, whether base station or mobile unit (handset).
A hitherto unsolved need has arisen for a diversity reception method and architecture which provides the advantages of OTD without undesirable limitations thereof.
One object of the present invention is to introduce diversity reception at the mobile unit in a manner overcoming a need to double the number of transmit channels at the base station.
Another object of the present invention is to provide a wireless communications air interface enabling reception of a base station signal by a mobile unit or handset equipped to carry out handset diversity in accordance with the principles of the present invention as well as enabling reception of a base station signal with a conventional handset.
A related object of the present invention is to provide a mobile unit or handset with a diversity reception capability employing a simple switching arrangement, thereby avoiding complexities in hardware and methods used in prior concurrent diversity reception approaches requiring plural receive channels in addition to plural antennas.
In accordance with one aspect of the present invention, a diversity handset is provided for use within a wireless communications network employing an air interface, such as CDMA. The network includes at least one base station operating in a transmit band for sending to the handset a pilot and Walsh coded traffic packets in plural time slots separated by separation time slots. The diversity handset includes a first antenna and a second antenna electrically separated from the first antenna in a predetermined way, such as by spatial separation of approximately one quarter mean wavelength, or by cross-polarization. An antenna switch switches a receiver between the two antennas. The receiver is most preferably a RAKE receiver responsive to the transmit band. The diversity handset also includes a time-of-arrival searcher circuit connected to the RAKE receiver to be responsive to the pilot for determining expected time of arrival of desired signals. The diversity handset also includes a controller for controlling the antenna switch based upon determined expected time of arrival of each time slot. First and second pilot integrators are connected to the RAKE receiver and are controlled by the controller. The first pilot integrator integrates pilot received via the first antenna. The second pilot integrator integrates pilot received via the second antenna. A pilot switch controlled by the controller switches between the first and second pilot integrators in synchronism with the antenna switch. A quadrature phase shift keying (QPSK) synchronous demodulator is connected to the RAKE receiver and is sequentially connected to said first and second pilot integrators by the pilot switch for synchronously demodulating received each slot of a traffic packet in accordance with integrated pilot corresponding to the particular antenna.
The foregoing objects and aspects of the present invention will be more fully understood and appreciated by those skilled in the art upon consideration of the following detailed description of preferred embodiments presented in conjunction with the accompanying drawings.